1. Field of the Invention
The invention relates to a method for precipitating or flocculating substances out of solutions.
2. Discussion of the Relevant Art
Undesirable ionic substances contained in water can be removed when they are transformed into the form of a sparingly soluble salt or mineral and are thus precipitated. Many metal ions, such as, for example, Ca2+, Mg2+, Fe2+, Me2+ ions can be precipitated in the form of sparingly soluble hydroxides. Such reactions can be controlled via the pH value.
Ca2+ ions in water are removed on a commercial scale in that they are precipitated as CaCO3 (calcium carbonate) (decarbonization) This reaction is also controlled via the pH value.
Closely related to the precipitation of substances contained in water is the term of flocculation and sedimentation. This is so because the removal of (precipitated) substances contained in water requires:that they can also be separated from the water. In the context of flocculation and sedimentation it is important how the precipitated products grow further and/or can conglomerate. The addition of certain salts (aluminum salts, iron salt) can control this behavior.
In the conventional method technology it is difficult to avoid a local overdosage when introducing the flocculation agent (for example, when adding sodium hydroxide solution or when dissolving sodium hydroxide pellets). A local overdosage can result in the precipitation of inherently less soluble substances contained in the water which then cause, as an entrained solid particle, component hard-to-control conditions in the subsequent precipitation process.
It is an object of the invention to provide an improved method for precipitation or flocculation of substances contained in solutions, especially water.
According to the invention, the method is characterized in that the solution is brought into contact with at least one ion exchange material which releases ions into the solution that effect precipitation or flocculation and/or has on its surface functional groups which catalytically effect flocculation or precipitation.
Ion exchange materials have been used in water or sewage treatment in order to exchange undesirable ions against desirable ions or ions that are less disruptive for the respective application purposes. Known are, for example, water softening devices which by means of ion exchangers bind Ca2+ and/or Mg2+ in exchange for Na2+ or H+ ions. Anionic exchangers (mostly in the Clxe2x88x92 or OHxe2x88x92 form) allow the removal of undesirable anions (NO3xe2x88x92, HCO3xe2x88x92 etc.) from the water. Known are also methods in which the Cu2+ ion or heavy-metal ions are removed by means of ion exchangers from the water. All these methods have in common that the ions removed from the water are bonded to the resin; once the capacity of the resin is depleted, it must be regenerated. During the regeneration process, the metal ions which have been concentrated can be, for example, removed from the regenerated compound.
Novel is now the idea to employ an ion exchange material for inducing a precipitation or flocculation process.
According to a first aspect of the invention, ion exchange materials are used as supports for the ions which in solution make the precipitation reaction possible. The component required for transformation of the ionic species to be precipitated into a sparingly soluble salt/mineral is provided by the ion exchange material which has been conditioned for this purpose. For example, an ion exchange resin of the OHxe2x88x92 form provides the required OHxe2x88x92 ions for a hydroxide precipitation in order to, for example, precipitate Fe2+ and Mn2+ in the form of hydroxides but of the water. Ag+ ions in the water can be precipitated as AgCl in the presence of an anionic exchange resin of the Clxe2x88x92 form.
When using specially conditioned ion exchange materials, the following advantages will result relative to the prior art in the field of water treatment:
The ion exchange material allows the directed addition of the components required alone for the precipitation reaction, for example, for the decarbonization of calcium carbonate-containing water. The principle of decarbonization of calcium carbonate-containing water is that the pH value is to be raised in order to shift the calcium carbonate/carbonic acid equilibrium such that the Ca2+ ions will precipitate in the form of calcium carbonate. Conventionally, the pH value increase is achieved by adding Ca(OH)2 NaOH and/or NaCO3. This addition has the disadvantage that with the OHxe2x88x92 or CO32xe2x88x92 ions acting as a base, additionally Ca2+ or Na30  ions are introduced into the water which partially counteract (additional Ca2 + ions which must be precipitated) or limit (sodium limit value in drinking water) the success of the method.
A weakly basic ion exchange resin of the OHxe2x88x92 form only releases OHxe2x88x92 ions; an ion exchange resin of the HCO3xe2x88x92/CO32xe2x88x92 releases only CO32xe2x88x92 and HCO3xe2x88x92 ions.
Better control of the precipitation process by avoiding local overdosage, especially in combination with a fluidized bed variant.
The possibility of controlling the precipitation process by means of the contact time of the water to be treated with the ion exchange material.
The ion exchange is a surface process and depends on the degree of loading of the ion exchange material with the ions required for the reaction and the type and concentration of ions in the solution which can be exchanged for the ions on the resin.
The contact time can be adjusted simply and can be changed optionally (by the size of the ion exchange resin bed and the flow-through amount in continuous operation; via the residence time in the reaction vessel (tank) during batch operation).
The recyclability of the ion exchange material.
Depleted ion exchange material, especially resins, can be removed easily from the reaction vessel or tank and regenerated. The regenerated material can then be returned into the process.
Ion exchange materials can be used as carriers of ions which control the flocculation in solution. In analogy to the above described mechanism, ions which enhance the flocculation of substances contained in the water (for example, Al3+ and Fe3+ ions) can also be brought into the corresponding solution by ion exchange from an ion exchange material (it is then required to provide an ion exchange material that is at least partially loaded with Al3+ and Fe3+ ions). All advantages are also applicable here.
For certain processes, the combination of dosage of pH-controlling ions (for example, anionic exchangers of the OHxe2x88x92 form) and flocculation agents (for example, cationic exchangers in the Fe2+ form) is expedient.
According to a second aspect of the invention, a specially conditioned ion exchange material can be used as a catalyst for precipitation of substances contained in water. In many real solutions there is the situation that the solution, when considered thermodynamically, is oversaturated with respect to a dissolved phase. Despite this fact, within a finite time period no precipitation takes place which would bring the solution into equilibrium. Such meta-stable solutions lack suitable growth locations where the precipitation could take place. Suitable growth locations are crystal seeds of the phase to be precipitated or special heterogeneous surfaces which decrease considerably the seed formation work and thus make the formation of crystal seeds in the range of low saturations possible. An example for such a solution is water which is oversaturated with respect to calcium carbonate.
It is known that biological systems (muscles, algae etc.) are able to initiate a directed crystal seed formation by means of certain functional groups. In particular, it was found that the carboxyl group of certain carboxylic acids (stearic acid etc.) induces calcium carbonate crystal seed formation. In regard to a mechanism of this reaction, it is assumed that carboxyl groups first bind Ca2+ ions from the water and that only this combination is able to induce the calcium carbonate crystal seed formation.
Ion exchange materials obtain their specific properties also as a result of certain functional groups: strongly acidic ion exchange materials carry as active functional groups, for example, the sulfonate group; weakly acidic ion exchangers have as active functional groups, for example, the carboxyl group (COOxe2x88x92).
When the carboxyl group of a weakly acidic ion exchange material is loaded by means of a loading process preferably completely with Ca2+ ions, this loaded material is suitable to catalytically form CaCO3 crystal seeds on its surface in aqueous calcium carbonate-containing solutions.
Such a conditioned weakly acidic ion exchange material can be used, for example, as a nucleus-forming agent and filter pellet in conventional decarbonization devices; and, furthermore, for the increase of the seed formation rate and thus the efficiency in the method and device described in the German patent application DE 19606633 A1. The contents of DE 19606633 A1 is included in the disclosure of the present application.
The catalytic efficiency depends on the bonding strength (electrostatic association) between carboxyl group and the Ca2+ ion: a bond which is too strong would not make possible the association of carbonate ions from the solution required for the seed formation; a bonding that is too loose would result in the loss of Ca2+ and thus in the destruction of the catalytic complexes. The electrostatic association of carboxyl groups and Ca2+ ions on the interface ion exchange material/water is affected by the electrical field on the interface. The catalytic efficiency of these specially loaded ion exchange materials is accordingly increased when they are, for example, applied to the electrodes described in the international application WO 95/26931 or produces them therefrom and, in this way, modulates or adjusts the functional groups by means of the described intrinsic electrical field. The contents of WO 95/26 931 thus is included in the disclosure of the present application.
The decarbonization of calcium carbonate-containing water via the directed dosage of OHxe2x88x92 ions via an ion exchange resin:
On a commercial scale, the decarbonization of calcium carbonate-containing water has been realized in that, by addition of certain chemicals (milk of lime, sodium hydroxide, soda), the pH value of the water was raised and thus the calcium carbonate/carbonic acid equilibrium was shifted greatly toward oversaturation. The resulting homogenous seed formation generated calcium carbonate crystal seeds on which the calcium carbonate dissolved in water then would precipitate (Mg2+ ions precipitate as Mg(OH)2).
The success of the method depends greatly on the type of process control.
The use of sodium hydroxide for increasing the pH value is a problem because at the location of addition of the sodium hydroxide an extreme pH value increase results locally which causes the precipitation of undesirable hydroxides. These hydroxides, for example, Ca(OH)2 colloids, are entrained as solid bodies into the process water and make the required pH value adjustment after the decarbonization process more difficult.
The problem when using milk of lime (Ca(OH)2) lies in the preparation of the solution to be added and the addition: it is practically impossible to produce a dosage solution which is free of Ca(OH)2 colloids. When these colloids are not completely dissolved in the decarbonization step, they present a great problem in the pH value reduction required subsequently.
The addition of milk of lime also adds further Ca2+ ions to the water which in the subsequently precipitation process are only partially precipitated also. Often, additional carbonate (in the form of soda-Na2CO3) must be added in order to be able to satisfactorily remove Ca2+ ions by precipitation. However, this also results in the undesirable increase of the Na+ contents in the water.
The goal of an optimal process control is furthermore a controlled seed formation rate: too many crystal seeds compete in regard to their growth and grow only to small calcium carbonate crystals which can be separated only with difficulty from the process water (sedimentation speed is too low, filtration is complex).
The use of a (strongly basic) anionic exchanger (for example, of the OHxe2x88x92 form) makes it possible to have an optimal process control.